Attenuation experienced by light propagating along a data-carrying link comprised of multimode optical fiber is dependent upon the number of “excited” spatial modes and the distribution of the optical power among these modes. More specifically, the outer (usually “higher-order”) modes are more subject to attenuation. Accordingly, if too much power is distributed in the outer modes when launching light in a multimode optical fiber, there may be excessive attenuation when light propagates along the optical fiber link.
Encircled Flux (EF) is a function (EF(r)) defined by international Standards, which characterizes the modal distribution of light in multimode optical fibers. It characterizes the near-field power distribution profile of light exiting (into air) a launch cable. It is defined as that proportion of the total exiting optical power which falls within a circle (i.e. “encircled”) of radius r at the end face of the fiber, where r is the radial distance from the optical center of the fiber core.
When performing insertion loss and attenuation measurements in a multimode optical fiber, the launch conditions of the test light must be carefully controlled in order to measure reproducible values of insertion loss or attenuation. If the test launch conditions are not well controlled, “differential mode attenuation” may lead to unrepeatable and irreproducible measurement results. If the launch condition of the test light is such that too many modes are excited (the modal distribution is then said to be “overfilled”), some modes, especially the outer modes, are more subject to attenuation. Conversely, if the modal distribution is “underfilled”, i.e. too few modes are excited, the attenuation is lower.
In order to address this issue, test and measurement international Standards such as the Telecommunication Industry Association (TIA-526-14-B) and the International Electrotechnical Commission (IEC 61280-4-1) define requirements on the modal distribution of test light for performing measurements on multimode optical fibers. For example, the IEC 61280-4-1 Standard provides for a target for the encircled flux function, EF(r) (see FIG. 1) characterizing the launch conditions and defines very tight tolerances on deviations from that target. More specifically, this Standard defines requirements based on lower and upper boundaries of EF values at four or five predefined radial values in the fiber core and for each of two wavelengths, i.e. 850 and 1300 nm.
When light is coupled to a multimode launch optical fiber, depending on the coupling conditions and on the optical power density of the light source, the coupling may result in the multimode launch optical fiber being “underfilled” (too few modes are excited) or “overfilled” (i.e. too many modes are excited). Means are required to adjust the launch conditions to comply with the EF requirements defined by the Standards.
A known method for controlling launch conditions is mandrel wrapping. Mandrel wrapping, i.e. the tight winding of the multimode fiber about a circular mandrel of a given diameter, results in a preferential attenuation of the high-order modes corresponding to an initially overfilled condition. Although the EF requirements as defined by Standards may be met using this technique, it has the drawback of being dependent upon the exact fiber parameters (i.e. the core diameter and the numerical aperture) of the multimode fiber used. The geometric tolerances provided by multimode-fiber manufacturers are typically not very restrictive and the core diameter of the actual launch-cable fiber therefore varies from one fiber spool to another, and often even within the same spool, within tolerances provided by the optical-fiber manufacturer. When a mandrel of predetermined diameter is employed to adjust the launch conditions within the very strict EF requirements, the only available free adjustment parameter is the number of turns about the mandrel. Unfortunately, different fractions of a turn are typically required on the last turn, resulting in a variability of the orientation of the fiber at the input or output of the mandrel. For that reason, this approach is particularly problematic in manufacturing conditions, where one may wish to subsequently encapsulate the launch conditioner in an optical module or incorporate it within a more complex instrument. This variability in the orientation of the fiber results in fiber management issues.
There is therefore a need for a modal distribution conditioner that addresses at least some of the above concerns.